Ionogenic compounds

Destructive solid sample preparation methods, such as digestion and mineralisation, are well known as they have been around for some time they are relatively cheap and well documented [13-15]. Decomposition of a substance or a mixture of substances does not refer so much to the dissolution, but rather to the conversion of slightly soluble substances into acid- or water-soluble (ionogenic) compounds (chemical dissolution). 

Chiou chose glyceryl trioleate (triolein) as model lipid because of its similarity to triglycerides which are abundant in organisms [109], Triolein is also a bulk lipid and the good correlation with the bioconcentration factor is restricted to neutral compounds of moderate hydrophobicity. No attempts were made to measure partitioning of ionogenic compounds with the glyceryl trioleate-water partition system. 

Jafvert, C. T., Westall, J. C., Grieder, E. and Schwarzenbach, R. P. (1990). Distribution of hydrophobic ionogenic compounds between octanol and water organic acids, Environ. Sci. Technol., 24, 1795-1803. 

Our final task in this chapter is to demonstrate how partition constants/coefficients can be used to calculate the equilibrium distribution of a compound i in a given multiphase system. As already pointed out earlier, for simplicity, we consider only neutral species. As we will see in Chapter 8, the equilibrium partitioning of ionogenic compounds (i.e., compounds that are or may also be present as charged species, as, for example, acids or bases) is somewhat more complicated to describe. However, the general approach discussed here is the same. 

Since the pH of the mobile phase will affect both the capacity factor and the selectivity, it is clear that manipulation of pH may be used to optimise separations of mixtures containing ionogenic compounds. At present nearly all stationary phases employed in reversed phase separations are unstable outside of the pH range of 2-8.5. 

The pH of the mobile phase has a strong influence on the solute retention and the peak shape of eluting ionogenic compounds. It is a critical factor for the reproducibility and ruggedness of a HPLC method (see also [67]). 

Ionic compounds of the type X"Y" or ionogenic compounds of the type. . . Y where the actual or potential anion Y is able to add to the carbon-carbon double bond to form a carbanion which can then propagate. Initiators of this kind include alkali metal alkyls, aryls, alkoxides and amides. The initiation step which occurs with potassium amide will serve to illustrate the mode of reaction of such initiators ... 

In Fig. 1, we have shown that the retention of an analyte may depend strongly on the pH of the mobile phase. In order to maintain reproducible pH values, one needs to use buffers in all pH ranges except at very acidic or strongly alkaline pH values. Buffers are solutions of ionogenic compounds that contain a conjugated pair of a proton donor and a proton acceptor. Thus, they stabilize the pH against the addition of small amounts of acid or base [6]. Let us discuss an acetate buffer as an example. This buffer contains an equimolar amount of acetic acid, the proton donor, and acetate, the proton acceptor. The pH of such a solution in water is 4.75. If one adds small amounts of acid or base (below the total buffer concentra-... 

Such a separation is shown in Fig. 10. We used a mixture of 13% acetonitrile and 4% methanol and an ammonium hydrogencarbonate buffer at pH 9.0. A short 5 cm XTerra MS Cjg column was sufficient for this separation. All peaks are cleanly separated. This example demonstrates the value of pH adjustment for the separation of ionogenic compounds. This method development strategy can be employed genericaUy for all separation problems involving ionogenic analytes. 

The ionogenic reaction which follows polymerisation, changes its kinetics from second to first order. It is completely inhibited if the solution contains more water than acid, though this does not affect the polymerisation itself. At the end of the polymerisation, the ionogenic reaction sets up an equilibrium which involves many components and which is slowly shifted by formation of stable species the position of equilibrium can be driven back towards predominance of non-ionic compounds by addition of fresh monomer. 

Determined as follows 1 mg of the copolymer was dispersed in a 0.1 ml solution of non-ionogenic surface-active compound at 37 °C for 10 min 0.2 ml of plasma were added to the suspension the mixture was incubated for 15 min at 37 °C 0.1 ml thrombin (5 U/ml) (thrombin time) or 0.2 ml of 0.025 M CaCl2 (recalcification time) were then added and the time of clot formation was measured. 

Some highly hydrophobic weak acids and bases exhibit substantial hydro-phobicity even in the ionized state. For highly hydrophobic ionogenic organic compounds, not only is transfer of the neutral species between the aqueous phase and the immiscible phase important, but the transfer of the hydrophobic, ionized, organic species as free ions or ion pairs may also be significant [37]. Mathematically, this is described by refining the n-octanol/ water partition coefficient, as defined in equation (2.7), to reflect the pH-dependent distribution between water (W) and K-octanol (O) of chemical X in both the ionized and nonionized forms. If chemical X is a weak acid, HA, the distribution ratio is... 

For example, the ratio of the n-octanol/watcr distribution coefficient of the nondissociated species to that of the ionic species is nearly 10,000 for 3-methyl-2-nitrophenol, but only about 1000 for pentachlorophenol because of the greater significance of the hydrophobicity of the ionized form of pentachlorophenol. The logarithm of the -octanol/water distribution coefficient of pentachlorophenol as the phenolate is about 2 (determined at pH 12, and 0.1 M KC1), which indicates significant distribution of the ionized form into the n-octanol phase [8,37], Extraction of such highly hydrophobic ionogenic organic compounds can result from mixed-mode mechanisms that incorporate both the hydrophobic and ionic character of the compound. 

Repeated extractions may be required to recover the analyte sufficiently from the aqueous phase. Neutral compounds can have substantial values of Ko. However, organic compounds that form hydrogen bonds with water, are partially soluble in water, or are ionogenic (weak acid or bases) may have lower distribution coefficients and/or pH-dependent distribution coefficients. Additionally, the sample matrix itself (i.e., blood, urine, or wastewater) may contain impurities that shift the value of the distribution coefficient relative to that observed in purified water. 

If the compound is ionogenic (or ionizable) in aqueous solution (as discussed earlier), there may be an electrostatic attraction between the... 

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